1. Field of the Invention
The present invention generally concerns electrophotographic imagers wherein a latent electrostatic charge image that has been optically formed on a photoconductive surface is developed by a developer solution that includes both a toner and a liquid carrier.
The present invention particularly concerns the production of a quality, and uniform, image in and by an electrophotographic imager regardless that the concentration of toner in the developer solution used by the imager should vary over time.
2. Description of the Prior Art
In electrophotographic imagers a latent electrostatic charge image is optically formed on a photoconductive surface, such as by the selective photoelectric discharge of certain regions of a charged photoconductive surface by a scanning laser light beam. The latent electrostatic charge image on the photoconductive surface is developed by a developer solution that includes both a toner and a liquid carrier. The developer solution, including the toner which is electrically conductive, is charged to an electrical voltage potential. When it is applied to the photoconductive surface upon which the electrostatic charge image has been formed, the toner is electrically attracted, and adheres, to the image region (only). The carrier portion of the developer solution is recycled. The developed image, typically upon the surface of a drum or a plate medium, may subsequently be used for contact printing.
In greater detail, a photoconductive surface of a plate or a drum medium is uniformly charged to a high voltage, typically several hundred volts. A latent electrostatic charge image is formed upon the photoconductive surface by selective discharge of regions of the surface by a light beam. The latent electrostatic charge image upon the medium's photoconductive surface remains charged to the high voltage, other regions are substantially discharged. The entire surface, including the regions thereof where the latent electrostatic charge image is present, passes proximately to a prime electrode that is charged to a voltage potential. It is at this location where the photoconductive surface is bathed in developer solution, electrostatically precipitating the toner from out of the developer solution to adhere to, and develop, the latent electrostatic charge image.
This voltage potential of the prime electrode is typically a few tens of volts. This voltage is intentionally made approximately equal to the latent voltage at the discharged regions of the photoconductive surface. Being so charged the prime electrode is still at several hundred volts potential difference from the region(s) of the photoconductive surface that are still charged, and where the latent electrostatic charge image exists.
The prime electrode that is charged to a few tens of volts is commonly constructed as one wall of an orifice, while a remaining wall portion of the orifice may be considered to constitute still another electrode. This remaining wall portion, or other electrode, is commonly at electrical ground. The orifice is typically in the shape of an elongate slot. The slot is as long as the medium is wide (typically eight inches or more), an inch or so in width, and of such thickness as provides a precision clearance on the order of a tenth of one inch (0.1") to a plate photoconductive medium that passes through it.
The nominal value of the electrostatic field that exists across the slot, and between its long walls, is some tens of volts magnitude, typically -25 v.d.c. In its regions that were depleted of charge (by an imaging optical laser beam that "writes white"), the photoconductive surface of the medium typically has a residual electrostatic charge of the same some tens of volts, or -25 v.d.c. Meanwhile, in the region(s) of the latent electrostatic charge image, the photoconductive surface of the medium typically has an electrostatic charge of some hundreds of volts, nominally -400 v.d.c. The side of the medium that is reverse to its photoconductive surface is typically metal (especially in the instance of a plate medium), and is typically at ground potential.
The medium having a selectively electrostatically charged photoconductive surface, meaning a surface that possesses a latent electrostatic charge image, is bathed, typically on both its sides, with liquid developer solution while in the presence of the prime electrode, and an electric field between itself and the prime electrode. Namely, this developer "bath" occurs where, and while, the medium passes proximately to the prime electrode, which is typically means through the slot, and within the (lessor}electric field of the slot.
The photoconductive surface of the medium faces the prime electrode which is, again, typically a side of the slot that is charged to a few tens of volts, nominally -25 v.d.c., voltage potential. Intentionally, and by adjustment, very little difference exists between this potential and the residual electrical potential of the charge-depleted regions of the photoconductive medium. Accordingly, only insignificant amounts of toner are electrostatically precipitated out of the developer solution onto these regions.
Conversely, the high voltage difference between the latent electrostatic charge image region(s) of the photoconductive surface and the side of the slot--nominally about -450 V.D.C. --(-25 v.d.c.) =-425 v.d.c.) causes a toner to be electrostatically precipitated (at a certain rate) out of the developer solution and onto this (these) region(s), blackening them. In other words, the regions of the photoconductive surface that were exposed by the (laser) light beam are "written write", and the unexposed regions were "left black" --although this can be reversed.
It is sometimes alternatively stated that the toner within the developer solution is "charged" to a voltage potential different that the latent electrostatic charge image region(s) of the photoconductive surface, and that it is "attracted" out of the developer solution. This is common, and acceptable, terminology. However, it is possibly more precise to say that the toner is electrostatically precipitated, or attracted, out of a developer solution in the presence of a (high voltage) electric field.
The reverse side of the medium that is opposite to its photoconductive surface, and which is typically at ground potential, does not experience a large electric field relative to the side of the slot that it faces, which slot side (or "other electrode") is typically also at ground potential. Neither does this reverse side even have a surface that is capable of holding toner. Accordingly, it remains substantially unaffected by the developer solution, and by the development of the latent electrostatic charge image upon the medium's photoconductive surface.
The quality, and uniformity, of the image that is printed by an electrophotographic imager varies with changes in the concentration of toner within the developer solution. This concentration varies over time as the toner is depleted during use of the imager. The electrophotographic imager is normally adjusted so that, with a fresh batch of developer solution having a maximum concentration of toner, the precise amount of developer solution that is applied to the photoconductive surface will be so as to develop the dots of a half-tone image with clarity, and without smearing.
If too much developer solution, or a developer solution of greater than maximum toner concentration, is applied to photoconductive surface then the developed image will not only be excessively dark, but will be fail to be satisfactorily clear and sharp. Conversely, as amount of toner within the developer solution becomes depleted, the black regions of the image generally get lighter over time. Eventually this will cause the image to become unsatisfactory.
The solution to toner depletion has, in the past, been straightforward. Either (i) a complete new batch of developer solution having a full concentration of toner is entered into service periodically, as needed, and/or (ii) the desired concentration of the toner within the developer solution is attempted to be periodically reconstituted by addition of pure, or a high concentration of, toner.
The first approach permits simplicity in the operation of the electrophotographic imager, but demands the periodic replacement of the entire developer solution. The initial concentration of toner relative to liquid carrier within a developer solution is typically less than 1:30, or &lt;3.3%. When the depleted developer solution is replaced the ratio of remaining toner to liquid carrier is typically better than 1:45, or &gt;2.2%. Accordingly, less than about one-half (1/2) the toner that was originally within the developer solution has been used. The replacement and discard of the depleted developer solution incurs an attendant (i) economic cost, (ii) labor cost and inconvenience, and (iii) environmental impact resultant from the disposition (even through approved channels) of the spent developer solution, which is caustic.
The second approach of periodically replacing lost toner from the developer solution also entails certain costs. Concentrated liquid toner material typically has a sludge-like consistency with a viscosity of 5,000 cps and greater. This material is not easy to handle, nor to dispense with accuracy. If the dispensing of this material is to be precise and reliable, special mechanisms are required. Even it the concentration of toner is maintained within desired ranges for extended periods, the developer solution will, over time, become spent due to the accumulation of contaminants, etc., and must be replaced in its entirety. When it is so replaced the toner is, by operation of the enhancement mechanism, normally at or near full strength. Accordingly, its replacement again entails a waste of toner. Additionally, maintenance personnel must separately provide both toner and carrier solutions (or at least toner and a developer solution) to the electrophotographic imager, compounding the consumable materials handling and storage problems.
One manner of controlling the concentration of toner within a developer solution consisting of toner and a liquid carrier is shown in U.S. Pat. No. 4,119,989 to Carvalko and Tolmie for a SYSTEM FOR CONTROLLING CONCENTRATION OF DEVELOPER SOLUTION. In the Carvalko and Tolmie system a source of light is directed through a developer solution and focused on a photosensor. The photosensor generates an output signal proportional to the light reaching it. Because the toner component of the developer solution is opaque (black) while the carrier component of the same solution is transparent (clear), this signal is proportional to the concentration of toner within the developer solution. The signal is compared to a reference signal that corresponds to a predetermined optimal toner concentration in order to produce an error signal. The error signal is used to modulate the amount of time that a servo mechanism is operated to add toner concentrate to the developer solution, thereby to control the concentration of toner within the developer solution.
Similarly, U.S. Pat. No. 5,003,352 to Duchesne et al. shows an automated system for both (i) controlling toner concentration within a developer solution, and (ii) dispensing the developer solution, within a high speed electrophotographic printing press (operating at speeds of 100 ft./min. and greater). The concentration of liquid toner that is dispersed within a solution of consisting of (i) sludge-like consistency liquid toner concentrate material, and (ii) a liquid carrier, is monitored by an electrical conductivity sensor. This monitoring of electrical conductivity will prove to be similar to the present invention. However, the monitored concentration is again, just as with the system of U.S. Pat. No. 4,119,989 to Carvalko and Tolmie, used to control the flow of the liquid toner concentrate material into the developer solution. The speed of the printing press is also measured, and is correlated with the predetermined optimal flow rate of developer solution to the printing station at different press speeds. The flow rate of the developer solution to the printing station is monitored by a flow meter, and is controlled in a feedback system including logic circuitry and an adjustable flow valve so as to continuously equal the predetermined optimal rate.
According to these problems regarding the formation of quality images in an electrophotographic imager during depletion of the toner within its developer solution; and the cost, complexity and wastage of previous systems for addressing these problems; it would be useful if an improved system could be devised. Such a system would desirably be (i) inexpensively implemented and (ii) simply maintained. It would (iii) continuously produce images of highest quality while (iv) reducing wastage, with its attendant economic and environmental costs, of consumable developer solution (in both its toner and liquid carrier components).